This invention relates to a coded voice signal transmission system.
One method that has heretofore been proposed for making effective use of a transmission line in a coded voice signal transmission system is a digital speech interpolation (DSI) system, in which the presence of a voice signal is detected so that only the detected voice signal is transmitted. This system merely utilizes a macroscopic characteristic of voice signal and its signal compression ratio is generally about 1/2 at most. With the combined use of this system and a system of removing a microscopic characteristic of the signal voice, for example, redundancy of the waveform of the voice, it is possible to make more effective use of the transmission line.
On the other hand, a method for removing redundancy of the voice waveform is a predictive encoding system and the typical one is a delta modulation system. This method periodically predicts mainly amplitude information. However, this method is effective in the case of relatively low quality transmission as compared with the conventional PCM system but is not effective in a public telephone circuit requiring high quality transmission characteristics.